Running at much faster paces won't harm your endurance, unless you neglect your endurance training.
There isn't really such a thing as a lactate threshold or an aerobic threshold.
So why do so many people talk about lactate threshold and aerobic threshold etc? Because they want to sound knowledgable.
Lactate is a product of carbohydrate metabolism, and the more carbs you have stored, the more latate you can produce and vice versa. So in a tempo run at a steady pace, you use more lactate as fuel at the beginning and gradually more fat for fuel, the further you run.